1. Field of the Invention
The present invention relates to a method of producing a semiconductor base material and, more particularly, to a method of producing a solar cell by depositing a thin-film crystal layer on a low-cost substrate.
2. Related Background Art
There is the known technology of forming a thin-film semiconductor layer on a porous layer formed on a semiconductor substrate, thereafter breaking the porous layer to separate the thin-film semiconductor layer from the substrate, and using the semiconductor substrate after the separation, again for the formation of a thin-film semiconductor layer. Since this technology permits effective utilization of materials, inexpensive thin-film semiconductors can be obtained with good quality. Methods available for the separation include chemical methods by etching, or physical methods making utilizing an action of ultrasonic wave, pulling force, or the like.
Japanese Patent Application Laid-Open No. 7-302889 describes a physical separation method in which an epitaxial layer is grown on a porous layer formed in a surface of a silicon wafer, another wafer is bonded to the epitaxial layer (silicon layer), and a pressure, shearing stress, ultrasonic wave, or the like is applied to the porous layer to effect separation. Japanese Patent Application Laid-Open No. 8-213645 describes another method in which a silicon layer is epitaxially grown on a porous layer formed in the surface of a single-crystal silicon substrate, the back surface of the single-crystal silicon substrate is bonded to a jig with an adhesive, thereafter the epitaxial layer is bonded to another jig with an adhesive, and then a pulling force is applied between these two jigs to obtain a thin-film semiconductor. Japanese Patent Application Laid-Open Nos. 10-189924 and No. 10-190029 describe that the back surface of the single-crystal silicon substrate is fixed to the jig by vacuum attachment (or chucking or suction), another jig is bonded to the epitaxial layer, and the two jigs are separated from each other to break the porous layer, thereby obtaining the thin-film epitaxial layer (and a solar cell using it).
For separating the thin-film epitaxial layer from the original semiconductor substrate by use of peeling force, holding of the substrate is an important factor. For obtaining the thin-film semiconductors at a low cost, the original semiconductor substrate has to be used repeatedly many times. Therefore, the vacuum attachment and/or electrostatic attachment is considered to be a suitable method for holding the semiconductor substrate without damage or contamination thereto, as described in Japanese Patent Applications Laid-Open No. 10-189924 and No. 10-190029. In the methods of initiating peeling from an edge of the substrate while holding the substrate by the vacuum attachment and/or electrostatic attachment, however, when the rupture strength of the porous layer is greater than a certain value, there are possibilities that the substrate-attracting force is defeated by the force necessary for the rupture of the porous layer, which is the pulling force for the separation of the thin-film semiconductor layer, so as to lift the substrate. When the wafer is separated with rolling the support member of the thin-film semiconductor up around a roller, there are also possibilities that the wafer is broken.
An object of the present invention is to solve the above-stated problems, thereby providing a method capable of obtaining a thin-film epitaxial layer with excellent characteristics in a good yield without lifting of the substrate and permitting repeated uses of the substrate.
The inventors have conducted intensive and extensive studies in order to solve the above problems and finally have accomplished the present invention.
According to the present invention, the substrate is held by vacuum attachment and/or electrostatic attachment and the separation of a thin-film epitaxial layer is initiated from an area except for the periphery of the substrate. By this method, the thin-film epitaxial layer with excellent characteristics can be obtained in a good yield and the substrate can be used repeatedly, thus permitting low-cost production.